The present invention relates to a process for production of (meth)acrylic acid, the (meth)acrylic acid obtainable by this process, a device for production of (meth)acrylic acid, the use of the device or of a purification device for the production of (meth)acrylic acid, (meth)acrylic acid, water-absorbing polymers, the use of water-absorbing polymers and fibers, sheets, foams and composites.
“(Meth)acrylic acid” is used in this text for the compounds with the nomenclature names “methacrylic acid” and “acrylic acid”. Of the two compounds, acrylic acid is exemplary both in connection with the process according to the invention as well as in connection with the chemical products according to the invention.
Chemical starting products and intermediate products, in particular starting products for polymer production, are currently produced in very large amounts. In order to satisfy increasing demands on quality, these products may contain practically no impurities. This is particularly the case for acrylic acid, if it is to be used for the production of water-absorbing polymers, which are, for example, used in hygiene articles. Impurities in the acrylic acid influence disadvantageously its polymerization. Thus, with increasing amount of impurity in the acrylic acid, not only the portion of residual monomers in the acrylic acid polymers increases, but also the absorption behavior of the water-absorbing polymers based on acrylic acid is disadvantageously influenced by impurities in the acrylic acid used.
Acrylic acid is commonly obtained by catalytic gas phase oxidation of propylene with an oxygen-containing gas. In this way, in a two-step process, the propylene is first oxidized catalytically to acrolein, which is then converted to acrylic acid in a second process step, likewise using catalysts. The thus-obtained acrylic acid is removed in the form of an aqueous solution from the gaseous reaction mixture by absorption with water. The water in the thus-obtained aqueous acrylic acid is then separated with formation of a crude acrylic acid, by distillation, for example by means of an entrainer, whereby an acrylic acid-comprising bottom product (i.e., crude acrylic acid) is obtained.
Methacrylic acid can be produced analogously to acrylic acid, likewise by catalytic gas phase oxidation, whereby in this case, C4-starting compounds, such as, for example, isobutene, isobutane, tert-butanol or methacrolein are used. The purification of the methacrylic acid obtained by the catalytic gas phase oxidation of C4-starting compounds comprises, as for the purification of acrylic acid, the absorption of the methacrylic acid in a suitable solvent and the subsequent separation of the solvent whereby in this case, a crude methacrylic acid is obtained.
Since both in the catalytic gas phase oxidation of the C3 or C4 starting compounds, besides the acrylic acid or the methacrylic acid, other oxidation products are also formed, such as, for example, maleic acid anhydride, which, in the case of the production of water-absorbing polymers based on polyacrylates, can inhibit the polymerization and have a disadvantageous effect on the absorption properties of the water-absorbing polymers, a further purification of the crude (meth)acrylic acid is necessary. The purification of the crude (meth)acrylic acid occurs by means of purification processes known from the prior art, for example, by means of distillation of crystallization, whereby in the case of crystallization, it is technically differentiated between the two processes of suspension crystallization and layer crystallization (Wintermantel et al., Chem. Ing. Tech. 1991, 63, 881-891; Steiner et al., Chem. Ing. Tech. 1985, 57, 91-102).
Crystallization has the advantage over distillation that impurities, which cannot be removed by distillation, can often be separated by means of crystallization. Furthermore, crystallization can be carried out at considerably lower temperatures in comparison to distillation, so that the extent of formation of (meth)acrylic acid dimers or (meth)acrylic acid oligomers during the purification process can be reduced. Crystallization has, however, the disadvantage that impurities, which may be present in the melt, in particular fumaric acid, maleic acid and maleic acid anhydride, also precipitate on crystallization. Since the mother liquor or the melt is depleted in (meth)acrylic acid during the crystallization, impurities in the mother liquor are constantly enriched. As soon as the solubility limit is reached, these compounds precipitate from the mother liquor in crystalline form. In the case of a static layer crystallization, these undesired crystals form on the floor or the walls of a crystallizer. Since the crystals stick to the surface, these impurities remain in the crystallizer when the uncrystallized melt is discharged. If the separated crystal layer is then melted, whereby commonly two or more fractions are formed, the precipitated crystalline impurities which are still present on the floor and on the walls of the crystallizer are again taken up by the warmed melt. The result of this is that these impurities can never be completely removed and concentrate in the crystallizer. In a large-scale purification plant, in the course of a day, considerable amounts of impurities, such as, for example, maleic acid, can accumulate, which can block the conduits and valves of the purification device for (meth)acrylic acid. Impurities can also condense as a solid coating on the crystallizer walls or on the floor. These deposits not only lead to blockages, but also effect a reduction of the capacity of a plant for purification of(meth)acrylic acid, since a considerable amount of the volume, for example of the tanks situated in the plant, is taken up by the precipitated crystals.
It has thus become a necessity to separate the deposits.
To this end, WO-A-00/45928 proposes to prevent the precipitation of impurities in the purification process for acrylic acid, by addition of a solvent or a solvent mixture, whereby the solvent or the solvent mixture is added in such an amount that the impurities are held in solution. Water may be used as solvent. The disadvantage of this process is, however, that maleic acid and maleic acid anhydride are not separated by this process from the mother liquor, but constantly enrich in the mother liquor. The dilution of the mother liquor with solvent or the solvent mixture has, in addition, the result that in subsequently attached crystallizers, a crystallization of the acrylic acid is made more difficult by the introduction of a solvent.
One aspect of the present invention is to provide a process with which the disadvantages described in the prior art could be overcome.
Another aspect of the present invention is to provide a process for production of (meth)acrylic acid, which comprises a purification process for (meth)acrylic acid, in which impurities, in particular fumaric acid, maleic acid or maleic acid anhydride, can be separated in a simple way, without making more difficult a subsequent crystallization of the (meth)acrylic acid.
A further aspect of the present invention is to provide a (meth)acrylic acid, which, in comparison with the (meth)acrylic acid described in the prior art, is characterized by a particularly low content in impurities, such as, for example, fumaric acid, maleic acid or maleic acid anhydride.
An additional aspect of the present invention is to provide water-absorbing polymers, which are characterized by a particularly low content in impurities, such as, for example, fumaric acid, maleic acid or maleic acid anhydride, and thus by a particularly advantageous skin tolerance.